1. Field of the Invention
The present invention relates to a printer designed to be able to correct registration of images of different colors rendered at wrong areas, and a method of correcting a color registration error.
2. Description of the Related Art
FIG. 1 illustrates a cross-sectional view of a general liquid electrophotographic color printer. The printer operates by transporting developer by photoreceptor belt 14 onto paper 23 passing through a transfer device 20.
Photoreceptor belt 14 follows a circulation path around a plurality of rollers 11, 12 and 13. Installed around the circulation path are a reset device 15, optical scanning devices 30, 40, 50, 60, optical detectors 38, 48, 58 and 68, developing devices 36, 46, 56 and 66, a drying device 18, and the transfer device 20.
Chargers 15b, 37, 47, and 57 charge the photoreceptor belt 14 with a predetermined potential to prime the belt for the registration of a new electrostatic latent image. Yellow, magenta, cyan, and black each require the registration of a new electrostatic latent image. After transfer of the color images onto paper, an exposing device 15a erases any remaining electrostatic charge from the photoreceptor belt 14.
The printing process begins when the leading edge of a designated image writing page area on the photoreceptor belt 14 reaches the scanning position of the first optical scanning device 30. The first optical scanning device 30 projects light corresponding to yellow-color image information onto the photoreceptor belt 14, which already carries a charge from charger 15b. An electrostatic latent image is formed on the photoreceptor belt 14 by the first optical scanning device 30, and is developed by the first developing device 36, which supplies yellow developer (Y).
The page area on the photoreceptor belt 14 is recharged by charger 37 so that the next electrostatic latent image can be written onto the page area. When the charged page area reaches the scanning position of the second optical scanning device 40, the second optical scanning device 40 projects light corresponding to magenta-color image information. Another electrostatic latent image is formed on the photoreceptor belt 14 by the second optical scanning device 40, and is developed by the second developing device 46, which supplies magenta developer (M).
This process repeats as the page area continues through the third and fourth optical scanning devices 50 and 60, projecting cyan-color and black image information, and through the third and fourth developing devices 56 and 66, supplying cyan-color developer (C) and black developer (BK). Consequently, by passing through all four developing devices, a full color image can be formed within the page area on the receptor belt 14.
The page area continues along the circulation path through the drying device 18, which removes liquid carrier from the photoreceptor belt 14.
The color image formed of developing material on the photoreceptor belt 14 is transferred to a transfer roller 21, which rotates in contact with the photoreceptor belt 14. The color image is transferred via the transfer roller 21 to a sheet of paper 23 entering between the transfer roller 21 and a fixing roller 22.
This printer is susceptible to color registration error. Color registration error occurs when the image of an individual color is not registered at the correct pixel writing position in relation to the color images of the other colors. This occurs as optical scanning devices 30, 40, 50 and 60 sequentially write their respective color image information onto the same page area on the circulating photoreceptor belt 14, when the respective color image information is inaccurately registered within the page area. As a result, the desired image cannot be obtained.
The conventional art corrects color registration error by printing an image of a predetermined test pattern, having an inspector measure the distance difference between adjacent test patterns of different colors output on the paper 23, calculating the amount of color registration error, and adjusting the printing process to compensate for the error. However, this method is complicated, since the inspector must manually calculate the color registration error. Manual inspection has a high possibility of introducing extraneous error and imprecision into the calculation of the color registration error amount.
In another conventional method, color registration is corrected by printing test patterns for each color using all of the developing devices 36, 46, 56 and 66 by a normal printing method, and calculating the amount of color registration error by detecting information associated with the interval between the printed test patterns of the different colors. Detection is performed by an image information acquisition sensor such as a charge-coupled device (CCD). However, this method requires the installation of image information acquisition sensors for the respective colors to accurately measure a color registration error, thus complicating the installation.